The broad goals of this Project, now entering its tenth year, are to develop the experimental and computational tools of electron cryomicroscopy (cryoEM) in the context of a widening range of biological applications. We seek in particular to connect electron microscopy (EM) with x-ray crystallography and to move molecular EM toward becoming a high-resolution tool. One of several advances during the past funding period has been to reach near-atomic resolution (<4A) for several virus structures, fulfilling a conjecture made 15 years ago by Henderson that it would be possible to image biological assemblies by cryoEM at this level of detail. We propose three principal themes for the coming project period. (1) Continued methods development. We will extend computational methods for near-atomic resolution structures to include images of multi-state single particles and helical assemblies (Grigorieff, Harrison); we will improve sample preparation for uniformity and homogeneity, building on the development of Affinity Grids during the last grant period (Walz); we will explore methods to reduce beam-induced movement (Grigorieff); resolution improvement for cellular imaging (Nicastro). (2) Electron cryotomography (cryo-ET) as a bridge between visualizing near-atomic resolution structures and studying their intracellular dynamics by optical microscopy and live-cell imaging (Nicastro, Harrison, Grigorieff). Rotavirus entry and clathrin-coat dynamics are two specific projects for which structures determined as part of this Project and results from live cell fluorescence microscopy raise mechanistic questions best answered by frontier methods in cryo-ET. (3) Analysis of transient and multi-state assemblies, including enhancements made possible by the methods developed as part of theme 1 (all four projects). In pursuit of this theme, we will focus especially on the large-scale organization of dynamic structures such as kinetochores, cilia, and transport-vesicle tethering complexes.